1. Field of the Invention
The present invention relates to ferromagnetic powders with an electrically insulating layer on iron particles intended for the manufacture of components having improved soft magnetic properties at low and medium frequencies. The invention comprises an iron powder coated with a dielectric insulating layer comprising boron (B) bearing compounds to form an insulated ferromagnetic powder. The present invention also relates to a method of making these insulated ferromagnetic powders. The present invention further relates to a method of synthesizing a product made from insulated ferromagnetic powders via a post-heat treatment at a moderate temperature (300° C. to 700° C.), to form a glass-like coating which acts as an electrical insulator. A preferred embodiment of the present invention is obtained when small amounts of alkali bearing compounds are added to the precursors to modify the coating chemistry and significantly increase the electrical resistivity after heat treatment.
This invention concerns the utilization, at low to medium frequency magnetic solicitations, of ferromagnetic materials such as elemental iron (Fe), nickel (Ni) and cobalt (Co) or other Fe, Ni and Co based magnetic alloys. Such materials are characterized by a high permeability μ, defined as the degree of magnetization of a material in response to a given magnetic field; Bmax, a specific saturation point (also designated “saturation induction point”) over which no further magnetic induction can be created in the material; and Br, a certain amount of residual induction remaining when the applied magnetic field is released.
The magnetic materials used in this invention are named soft magnetic as opposed to hard magnetic materials as they can be easily magnetized and demagnetized by an external magnetic field. The magnetic induction created in these materials when magnetized can be reduced to zero by applying a reverse magnetic field of strength Hc, known as the coercive field. Broadly, hard magnetic materials are those with coercivities above 10 kA/m while soft magnetic materials are those with coercivities below 1 kA/m. Note that properties like coercive field, remanent induction and permeability are strongly dependent on the condition of the material. These properties are affected by such factors as heat treatment or deformation whereas maximum induction is mainly determined by the nature of the soft magnetic material (e.g., composition, density).
When magnetically solicited, a fraction of the energy used to magnetize a soft magnetic material is dissipated in the form of thermal energy. At low frequency, hysteresis phenomena, defined by the irreversible variation in the magnetization associated with a change in the magnetic field strength, dominate and are responsible for most of the core losses. However, as the frequency of solicitation increases, eddy currents are created in the soft magnetic material. This, in turn, generates a magnetic field opposed to the applied external magnetic field. These eddy current losses result in a decrease of the effective cross-section of a part that can be magnetized. Eddy current losses can be decreased by either increasing the material resistivity or by splitting the magnetic circuit into smaller parts (e.g., laminations, particles).
2. Description of Related Art
Insulated Ferromagnetic Powders
Iron-based powders are currently used in conventional powder metallurgy to elaborate structural parts using compaction and high temperature sintering operations. A majority of these parts are used in the automotive industry due to their mechanical attributes. In the last several decades, metal powders have also been used to elaborate sintered components in which the magnetic property of the material is functionally important. Applications in this field include hard as well as soft magnetic devices normally intended for direct-current (DC) magnetic solicitation.
More recently, soft magnetic powder materials for low to high frequency applications have been developed. These materials consist of iron-based powders insulated by an organic or inorganic coating which limits eddy current losses when frequency increases. These insulated ferromagnetic powders are also consolidated using a compaction step but, in contrast to the standard powder metallurgy practice, they are not sintered. Instead, insulated ferromagnetic powders are heat-treated at temperatures lower than 700° C. in order to stress-relieve the parts without creating a metallurgical bonding between particles.
The resulting insulated ferromagnetic powder parts, also known as soft magnetic composites (SMC), are intended to replace electrical sheets. Electrical sheets present some drawbacks. One such drawback is that the punching step may induce mechanical stresses which decrease the magnetic properties of the material. This operation produces a certain amount of scrap. It is also usually required that the punched laminates be deburred in order to remove the sharp edges. Furthermore, depending on the sheet thickness and material quality, core losses and acoustical noise increase significantly at higher frequency. Finally, electrical sheets can only carry the magnetic flux in the 2D plane of the sheet thus limiting feasible machine designs.
Advantages of SMC over the electrical sheets are the following. First, because of their isotropic nature, the magnetic flux can also be carried in the third dimension thus allowing designers to decrease the outside diameter of the electrical motors while maintaining a similar torque and efficiency. Second, due to the reduction of eddy current losses, SMC materials have lower total core losses than electrical steels at a frequency higher than about 400 Hz to 1000 Hz, depending on the comparative material. Finally, the powder metallurgy process route allows the production of complex part shapes with no material loss while simultaneously minimizing the number of production steps.
Despite its advantages, SMC materials present some limitations. When compared to electrical steels, the maximum magnetic permeability of SMC is relatively low and makes them less preferable in applications where permeability is critical. Examples of lesser critical permeability applications are permanent magnet electrical motors in which a large air gap already exists between the stator and the rotor, to accommodate the magnets. Furthermore, SMC have higher hysteresis losses than electrical steels, thus limiting their use at low frequency (e.g., 60 Hz). Finally, because of their porous nature and non-sintered state, these materials have a lower mechanical strength than laminated materials.
Dielectric Coating
A critical parameter and main challenge in the fabrication of insulated ferromagnetic powders is the surface coating of the iron particles. The surface coating should be as thin as possible in order to minimize the attenuation of the magnetic flux while also presenting maximum dielectric properties. These dielectrics are categorized in two groups: organic and inorganic coatings. Organic coatings normally offer a higher mechanical strength but the allowable stress-relief temperature is limited to about 300° C. Inorganic coatings allow the SMC material to be heat-treated at higher temperatures thus maximizing the material magnetic properties.
Inorganic types of coatings have been developed in the form of borate glass-like coatings in order to maximize the magnetic properties of resulting materials. The dielectric properties of these inorganic glass-like coatings are highly dependent on their composition. In glass science, it is well known that the glass DC electrical conductivity is related to the ionic charge. For example, borates and phosphates naturally form negatively charged glasses, which can be changed using network modifier elements. These network modifiers, usually present in the alkaline and alkaline earth groups, interact with the glass former elements and decrease their network cross-linking. This phenomenon decreases the glass transition temperature and, furthermore, neutralizes the glass ionic charge thus limiting current transportation.
Processing of SMC Parts
Processing conditions of SMC materials strongly influence their final magnetic properties. For instance, in order to maximize the magnetic induction in such SMC materials, compacting pressures can be increased and the tooling heated to maximize the final density thus providing a direct effect on the magnetic induction. An important step in the processing is the post heat treatment which relieves partly the stresses induced in these materials during the compaction step. This stress-relief heat treatment can be done in air and, ideally, at the highest possible temperature in order to decrease the material coercive field while maintaining a relatively high electrical resistivity (to avoid metallurgical bonding between particles). Also, depending on the solicitation frequency of the application, the amount of dielectric and powder particle size can be adjusted. For instance, as frequency increases, finer particles and higher amounts of dielectric are required in order to minimize core losses. However, this will negatively affect the maximum permeability.
Electrical Applications
As mentioned earlier, this invention is intended for low to medium frequency soft magnetic applications. Examples of applications suitable for SMC materials include, but are not limited to, DC motors (with brushes or brushless), alternating-current (AC) motors (induction and synchronous), transformers, inductors, linear motors and voice coils.